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1. Field of the Invention
The present invention pertains to the following areas of technology: Apparelxe2x80x94Guards and Protectors; for wearer""s head and face; eye shields such as goggles having a lens-cover plate; and windshield covers.
2. Description of the Prior Art
Face shields are employed in environments where contamination of the eyes may occur. It is well known in the art that flexible transparent lenses affixed by numerous methods are overlaid on the face shield for protection. The lenses are easily removed and discarded when visibility is reduced from the accumulation of dirt or other contaminants. In motor sports for instance, multiple layers of transparent lenses are overlaid on the face shield, each being sequentially removed as they become contaminated, because they reduce the visibility of the operator. The drawback of the lenses in the prior art is that each transparent lens applied over the face shield is itself a hindrance to good visibility due to its optical index of refraction. Most common materials used as plastics have optical indexes of refraction ranging from 1.47 to 1.498. The index mismatch between the removable lens and air (air has an optical index of 1.00) causes a reflection of 4% of the light that would normally come to the operator""s eyes. This reflection effect is additive for each additional surface to air interface. Then for each removable lens having two surfaces, the reflections are 8%. Thus a stack of seven lenses would reflect 42% of the light away from the operator thereby reducing the brightness of the objects viewed. A second optical phenomenon occurs simultaneously that also reduces visibility. The reflections are bi-directional and thus make the lens stack appear as a semi-permeable mirror to the operator. This mirror effect further reduces visibility, because the light that passes through the lens stack reflects off of the operator""s face and then reflects off of the lens stack into the operator""s eyes. The effect to the operator is that he sees his own image on the inside of the stack nearly as brightly as the objects viewed on the outside. This significantly reduces visibility.
Another drawback to this stacking arrangement is that moisture exhaled by the operator""s breath can cloud or fog-up the lenses also reducing visibility. The air spaces between each lens allows the moisture to enter this area.
An object of the present invention is to provide a series of easily removable optically clear lens stacks that do not cause reflection to the operator""s eyes. The prior art discloses reflective lens stacks that do cause reflections to the operator""s eyes. An example of this type of prior art of reflective lens stacks is disclosed in U.S. Pat. No. 5,592,698 issued on Jan. 14, 1997 to Woods.
Refraction is the change in the direction in which waves travel when they pass from one kind of matter into another. Waves are refracted (bent) when they pass at an angle from one medium into another in which the velocity of light is different. The amount that a ray of a certain wavelength bends in passing from one medium to another is indicated by the index of refraction between the two mediums for that wavelength. The index of refraction indicates the amount that a light ray bends as it passes out of one substance and into another. When light passes from air to a denser substance, such as Mylar film, it slows down. If the light ray enters the Mylar film at any angle except a right angle, the slowing down causes the light ray to bend at the point of entry. This bending is called refraction. The ratio of the speed of light in air to its speed in the Mylar film is the Mylar film""s index of refraction.
The present invention includes a series of alternating optically clear films whose indexes of refraction are matched to within 0.2 and which will nearly eliminate all reflections to the operator""s eyes. The layers of film are adhesively laminated to one another and are compliant so there is no air between the layers. The film layers can be large and generally rectangular in shape with a tab extending from each of the film layers. The tabs can be staggered so that the user can remove the top most layer and then the next succeeding layer. This embodiment of the present invention can be applied to race car windshields, windows, visors or direct view displays such as ATM machines that are subject to contaminating environments. Accordingly, the present invention is an adhesively laminated multi-layered clear film adapted to be used on a racer""s face shield, or on the windshield of a race car to keep the viewing area clean during the course of a race.